A NEW TECHNIQUE FOR COMPENSATING THE SOFT TISSUE ARTEFACT AT THE UPPER-ARM: IN VITRO VALIDATION

Soft tissue artefact is the dominant source of error in human movement analysis whenever this is carried out using systems based on skin-mounted markers. At the upper-arm, the most corrupted measure is the humerus internal-external rotation: the aim of this work is to propose a new technique for compensating the artefact affecting this measure. The technique is based on the definition of a humerus bone-embedded frame (H2) almost "artefact free" but influenced by the elbow orientation in the measurement of the humeral axial rotation, and on an algorithm designed to solve this kinematic coupling. The algorithm was tested in vitro correcting H2 distortions during different tasks imposed to a mechanical model of the upper-limb; in the most general case of a motion involving all the degrees of freedom of shoulder and elbow, the application of the algorithm reduced the root mean squared error between the known and the measured axial rotation from 8.60 degrees to just 0.12 degrees. By means of the algorithm, therefore, H2 becomes a reliable humerus reference system and its future application for in vivo artefact compensation appears promising.